For example, as shown in FIG. 1, the active matrix type liquid crystal display device uses the thin-film transistor (TFT) 4 as a switching element, and is composed of firstly TFT substrate (TFT array substrate) 1 attached with transparent electrodes (picture electrodes) 5 and a wiring section 6 covering gate wiring and source/drain wiring, secondly opposing substrate 2 attached with common electrodes 7 and placed with a predetermined distance apart from and on the opposite side of the abovementioned TFT substrate 1, and thirdly a liquid crystal layer 3 filled between the TFT substrate 1 and the opposing substrate 2.
As for the above transparent electrodes 5, for example, ITO film of indium oxide (In2O3) containing tin oxide (SnO) at approximately 10% by mass, or IZO film of indium oxide (In2O3) containing zinc oxide (ZnO) at approximately 10% by mass is used.
FIG. 2 illustrates an enlargement of the ambits A in FIG. 1 in which the wiring section 6 electrically connecting to the transparent electrode 5 is included. For the gate wiring 26 in FIG. 2, a wiring structure made up with single-layer film of Mo or Cr or aluminum alloy film such as Al—Nd laminated with high melting point metals [molybdenum (Mo), chrome (Cr), titan (Ti), tungsten (W), etc.] has been adopted in the past. For the source wiring 28 or the drain wiring 29 (these wirings 28 and 29 are to be collectively referred to as “source/drain wiring” hereinafter), it has been also practiced conventionally to use a laminated wiring structure of single-layer pure aluminum (Al) combined with the above-mentioned high melting point metals. (See the patent documents 1, 2 and 3, for example.)
The reason why the above high melting point metals are used in lamination is as follows. If the above transparent electrode (ITO film) 5 is directly connected with the pure aluminum film or the Al—Nd or other aluminum alloy film composing the source/drain wiring, oxidization of aluminum may produce highly resistive aluminum oxide in the contact interface between the transparent electrode and the above pure aluminum film or Al—Nd or other aluminum alloy films, causing increase in contact resistance between the signal wire and the transparent electrode 5 leading to deterioration in display quality on the screen.
The above reason is also affirmed by the following facts. Aluminum is an easily oxidizable element for which aluminum oxide covering is easily formed in the atmosphere. Particularly, the oxygen used for making film of the transparent electrode 5 composed of metallic oxide or generated during the film-making process has a promotive effect on formation of the abovementioned highly resistive aluminum oxide covering.
To solve the aforesaid problem, barrier metals (high melting point metals) have been used as materials for lamination in the past, since they have been believed to have a good effect on preventing oxidization from taking place in the surface of the aluminum alloy wiring (alloy film) and enhancing desirable state of contact between the aluminum alloy wiring (alloy film) and the transparent electrode.
To build up the structure in which barrier metals are interventional, it would be needed to add a process of forming the barrier metals. Also the sputtering apparatus used for deposition of the gate wiring and the source/drain wiring would require an additional deposition chamber for the barrier metals. However, as higher-volume production enables cost reduction to advance for liquid crystal displays, etc., a new problem has emerged in that the above formation of barrier metals has been bringing about increase in manufacturing cost and aggravating productivity. The tendency in recent years is to require the kind of electrode materials and related manufacturing processes that can do without the barrier metals. In this connection, the inventors of the present invention have already proposed a plan aiming at simplifying the above process for formation of barrier metals and, instead, adopting an aluminum alloy film for wiring that can allow the transparent electrode to be connected to the wiring section directly. (See the patent document 4.)
Incidentally, the forming temperature for the gate dielectric film 27 formed following the gate wiring 26 is the highest among the processes for array forming of the thin-film transistors, and the above gate wiring 26 is subject to a thermal history of high temperature. Therefore, this gate wiring 26 is required to have a better heat resistance property than the source/drain wiring 28 & 29 which is formed in the back-end process. For this reason, aluminum alloys or the above high melting point metals, which possess higher heat resistance property than is required for the source/drain wiring 28 & 29, have often been used for the gate wiring 26. (See the patent document 5.)
Although a superb heat resistance property can be secured for the aluminum alloy with a large alloy content or the above high melting point metals, these materials still leave some other problems such as high electrical resistivity inherent in these wiring materials.
FIG. 3 shows the relationship between temperature (heat treatment temperature) applied to the aluminum alloy film and electrical resistivity. As shown in this FIG. 3, electrical resistivity is dependent on temperature; that is, the higher the temperature is, the lower the electrical resistivity will be. This is because, if the substrate is heated during making of the film, the alloy content is separated from the aluminum alloy film at a low temperature and, at the same time, recrystallization of aluminum progresses.
Even if alloy content is increased for more resistance to higher temperature in using an aluminum alloy film composing the gate wiring 26 which is subject to high temperature, the electrical resistivity decreases, as shown in the above FIG. 3, under the high-temperature state during formation of the gate dielectric film 27. However, if the above aluminum alloys or high melting point metals are adopted for the source/drain wiring 28 & 29 which is not subject to high temperature, electrical resistance cannot be reduced. Materials for the above source/drain wiring 28 & 29 have thus been studied about centering on the materials for which priority is given to electrical resistivity than to heat resistance.
When using different materials for the gate wiring and for the source/drain wiring respectively, however, it would become necessary to prepare plural kinds of wiring materials and complex apparatuses, which situation would not be appropriate for complying with the need for more efficient quantity production of, for example, liquid crystal displays, etc. Desired here, therefore, is realization of a thin-film transistor substrate for which still more simplified manufacturing process will do.    [Patent Document 1]    Japanese Patent Application Laid-open Publication No. 4-20930-A    [Patent Document 2]    Japanese Patent Application Laid-open Publication No. 6-12503-A    [Patent Document 3]    Japanese Patent Application Laid-open Publication No. 2001-350159-A    [Patent Document 4]    Japanese Patent Application Laid-open Publication No. 2004-214606-A    [Patent Document 5]    Japanese Patent Application Laid-open Publication No. 7-45555-A